Quantifying organic matter fluxes in an era of wetland instability

In an era of sea-level rise and altered sediment supply, unstable tidal wetlands may disintegrate and become a significant source of particulate organic matter and carbon to the coastal ocean. Quantifying the fluxes of particulate matter over tidal, seasonal, and annual timescales is critical for evaluating the ecosystem services provided by tidal wetlands. Using high-temporal resolution measurements over a 2.5 mo period we calculated suspended-sediment, organic matter, and particulate organic carbon (POC) fluxes in two tidal wetland systems adjacent to Chesapeake Bay. One system is characterized by several decades of instability and marsh collapse, while the other has remained stable over the same period. We collected >100 water samples over several tidal cycles and obtained a linear relationship between organic and mineral content. A subset of those samples was processed for carbon concentration and isotopic characterization; both systems displayed a consistent carbon fraction of ~40% of organic material. Isotopic analysis revealed a consistent d¹³C of -25 at both sites, consistent with a C3 plant source (likely Schoenoplectus). Ultimately the unstable wetland exported POC at a rate of 48 g/m²/y while the stable wetland imported 21 g/m²/y. While these rates are comparable to carbon burial estimates (48-105 g/m²/y), systemic erosion will continue to reduce the carbon sequestration potential of wetland systems while releasing POC to the coastal zone. Future assessments of wetland carbon sequestration potential must consider both wetland stability and export of POC from eroding systems.